1. Field of the Invention
My invention relates generally to the detection of voiceband signaling tones sent by a network component to a subscriber device or sent by a subscriber device to a network component. More particularly, my invention relates to methods and apparatus for reliably detecting voiceband-signaling tones in the presence of near-end and far-end speech.
2. Description of the Background
The Public Switched Telephone Network (PSTN) has long used combinations of discrete voiceband frequencies to carry signaling tones between an analog subscriber device and a serving switch. A common example is touch-tone-dialing where signaling tones are sent from a subscriber device to a tone detector on the serving switch to initiate a phone conversation. Other services, such as Calling Identity Delivery on Call Waiting (CIDCW), Analog Display Services Interface (ADSI Services), and Calling Number Delivery are initiated by a serving switch or service node sending voiceband signaling tones to a detector on a subscriber device.
Voiceband signaling tones, whether initiated by a serving switch, a service node, or a subscriber device, are best transmitted under controlled conditions where extraneous xe2x80x9cnoise,xe2x80x9d such as near-end and far-end speech, music, etc. (hereinafter collectively referred to as speech) is not present and therefore cannot interfere with the recognition of these tones. For example, a subscriber""s handset is muted while entering touch-tone digits, and services such as Calling Number Delivery occur while the subscriber device is in the on-hook state.
However, an increasing number of applications have emerged where voiceband signaling needs to occur under uncontrolled noisy conditions where near-end and far-end speech can mask a signaling tone or imitate a tone. As a result, the services these tones are intended to activate either fail to activate or are falsely activated. Such service reliability problems directly impact customer satisfaction and therefore service marketability.
For example, CIDCW and ADSI services require tone detection at a subscriber device and can occur asynchronously at any time during a call. These services are initiated by a serving switch/service node sending a CPE Altering Signal (CAS signal) to a subscriber device thereby triggering the device to temporarily exit voice operation and enter data mode operation. Because these services can be activated any time during a call, the CAS detector memo within the subscriber device needs to remain active for the duration of the call and must successfully discern a valid CAS signal from any speech present on the interface emanating from the near-end or far-end. Similar issues exist for services such as voice-mail, where tone detection at a network device is required.
The imitating and masking of signaling tones is formally referred to as xe2x80x9cTalkoffxe2x80x9d and xe2x80x9cTalkdownxe2x80x9d. Talkoff occurs when near-end or far-end speech erroneously triggers the signaling tone detection system (i.e., the tone detector accepts an imitation signal). Talkdown occurs when a signaling tone is sent but the signaling tone detection system fails to recognize the tone because it is masked by near-end or far-end speech (Note that for the remainder of this discussion I assume that far-end talkdown is not an issue because the device sending the tone typically mutes the far-end prior to transmission.).
Talkoff and talkdown are difficult to mitigate because the two issues simultaneously exist in many environments and they have an inverse relationship. To successfully deal with talkoff, a tone detection system must resist imitation signaling tones produced by speech by rejecting xe2x80x9cnon-purexe2x80x9d signals. To successfully deal with talkdown, a tone detection system must recognize weak signaling tones in the presence of speech and therefore accept xe2x80x9cnon-purexe2x80x9d signals as valid tones. As a result, good talkoff performance is usually achieved by sacrificing talkdown performance, and good talkdown performance is usually achieved by sacrificing talkoff performance.
The difficulty in mitigating talkoff and talkdown is illustrated by the concept of xe2x80x9cguard-action,xe2x80x9d which is the typical means utilized by signaling tone detection systems to deal with talkoff. Under this concept, a tone detector measures a xe2x80x9csignal-to-guardxe2x80x9d ratio to determine the purity of a signal. Signal-to-guard ratio is the ratio of the power present at a signaling-tone-frequency to the power of speech in one or several designated xe2x80x9cguard-bandsxe2x80x9d. Guard-bands essentially provide a relative measure of the speech present on a line. Talkoff is circumvented by requiring a large signal-to-guard ratio because a large ratio ensures that a detected tone is relatively pure with respect to the guard-band(s) and is not a simple imitation that will trigger the detector. Talkdown, on-the-other-hand, is circumvented by requiring a small signal-to-guard ratio because a small ratio ensures that an actual tone signal is not lost among the interfering speech. As a result, good talkoff performance creates talkdown because a valid signaling tone superimposed on speech may fail to satisfy the talkoff signal-to-guard ratio requirement. Good talkdown performance creates talkoff because an imitation signaling tone created by speech can easily satisfy the lax talkdown signal-to-guard ratio requirement.
Prior-art tone detection systems have been prone to reliability problems because rather than eliminate talkoff and talkdown, they have attempted to simultaneously balance/address talkoff and talkdown. This balancing ultimately leads to tradeoffs and thereby compromised performance of the overall tone detection system and the associated services that it supports.
Tone detection within a subscriber device, such as a phone or caller-id box, will be used as the basis for the remainder of this discussion. Nonetheless, my invention is also applicable to tone detection within a network device. As a base reference, FIG. 1 depicts a subscriber device 100 common to the prior-art and my invention. Subscriber device 100 comprises: (1) 2-wire access interface 108, which interconnects subscriber device 100 to serving switch 120 and service node 122; (2) hybrid 104, which converts the 2-wire access interface to a 4-wire access interface consisting of send path 101 and receive path 102; (3) balance network 106, which reduces the reflection of near-end speech from send path 101 on receive path 102; and, (4) other device components 110, which are not related to the operation of my invention, but are common in many communication applications.
FIG. 2 illustrates a first prior-art system where tone detector 201 is placed across 2-wire access interface 108. In this location, tone detector 201 must simultaneously deal with talkoff and talkdown and therefore balance the two issues, inherently leading to non-ideal detection performance. More important, this balance is extremely difficult and expensive to achieve because no effort has been made to reduce the strength of the near-end voice incident upon the tone detector and the detector must therefore balance two extremesxe2x80x94significant guard-band to resist near-end talkoff and significantly reduced guard-band to prevent near-end talkdown.
FIG. 3 illustrates a second prior-art system where tone detector 301 is placed across receive path 102 of the balance hybrid. Advantageously, this solution uses the transhybrid loss to attenuate the strength of the near-end speech incident upon the tone detector thereby addressing both near-end talkoff and near-end talkdown. However, due to the complexity of speech and the difficulty in achieving a perfect match between the balance network and the impedance presented by the local-loop interface, the hybrid does not completely eliminate the near-end speech. Specifically, because network impedance can vary widely depending on loop composition, loading, switch type, and the presence of any parallel subscriber equipment, hybrid 104 can produce less than 2 dB of transhybrid loss. As a result, a tone detector designed to balance talkoff and talkdown performance at a good transhybrid loss may function poorly under conditions when the hybrid is not providing sufficient loss. If the designer were to attempt to strike the performance balance at the worst case transhybrid loss of 2 dB, the benefit that was to be provided by this scheme would be essentially lost.
FIG. 4 illustrates a third prior-art system where two identical tone detectors, 401 and 402, are placed across receive path 102 and send path 101 respectively. The two tone detectors are connected through logic circuit 403, which controls the triggering of a tone based on the output of the two detectors. Advantageously, this solution removes near-end talkoff because logic circuit 403 remains idle whenever tone detector 402 registers a tone. Logic circuit 403 registers a tone only when tone detector 401 alone registers a tone, as shown by Table 1 below. One drawback of this solution is that any feedback of the alerting signal from other subscriber device components, such as a speakerphone, can cause detector 402 to activate and cause a valid signaling tone to be rejected. A second drawback is that this solution, like FIG. 3, does not provide any additional help to block near-end speech incident upon tone detector 401 and as a result, tone detector 401 must continue to balance/tradeoff far-end talkoff and near-end talkdown performance.
FIG. 5 illustrates a forth prior-art system where echo canceller 502 is inserted between send path 101 and receive path 102. Unlike the above solutions, this solution offers significantly better near-end talkoff and near-end talkdown performance by highly attenuating near-end speech incident upon tone detector 502. As a result, tone detector 502 need only address far-end talkoff and hence, little balance/tradeoff needs be made. The problem with this solution however is that it is quite expensive as compared to the cost of the typical subscriber device.
It is desirable to have methods and apparatus that overcome the disadvantages of prior-art systems and provide for the reliable detection of voiceband signaling tones that occur in the presence of near-end and far-end speech. With the exception of the system shown in FIG. 5, prior-art tone-detection systems have been problem-prone because they have attempted to simultaneously balance the effects of talkoff and talkdown leading to tradeoffs and thereby non-ideal system and service performance. Other prior-art systems, such as the system shown in FIG. 5, have attempted to eliminate talkoff and talkdown but at an expensive cost. Similar to the prior-art, my invention uses a tone detector across receive path 102. However, rather than balance the effects of near-end and far-end talkoff and talkdown, my invention overcomes the shortcomings of the prior-art and reliably detects voiceband signaling tones by selectively notching near-end speech energy and actively adjusting the signal-to-guard ratio based on the presence of near-end speech energy.
Specifically, my invention overcomes near-end talkoff through the use of a notch-filter inserted in send path 101. This notch-filter attenuates/removes from the near-end speech one or more of the frequencies that comprise the signaling tone to be detected by the tone detector. As such, any non-notched signaling components produced by the near-end speech that pass through the hybrid and appear on receive path 102 are xe2x80x9cinvalidxe2x80x9d with respect to the tone detector. As a result of the notch-filter, near-end talkoff should be practically eliminated.
My invention overcomes the issues of near-end talkdown and far-end talkoff by utilizing two separate signal-to-guard ratios within the tone detector: a large signal-to-guard ratio to combat the effects of the far-end talkoff and a small signal-to-guard ratio to combat the effects of the near-end talkdown. My invention switches between the two signal-to-guard ratios by predicting whether the near-end or far-end is speaking. This prediction is made by monitoring the presence of near-end speech through the use of a speech energy detector inserted in send path 101 and by utilizing the fact that the typical xe2x80x9cconversationxe2x80x9d alternates between far-end and near-end parties. Based on this prediction, my invention correspondingly switches the tone detector to a large signal-to-guard ratio to combat the effects of far-end talkoff or to a small signal-to-guard ratio to combat the effects of near-end talkdown. Unlike the prior-art, my tone detector behaves like two detectorsxe2x80x94a low guard detector that resists talkdown when the near-end is talking (and cannot be talked-off by the near-end due to the notch-filter) and a high guard detector that resists talkoff when the far-end is talking and the near-end is quiet. As a result, there is no need to force a single balance between talkoff and talkdown that can ultimately lead to non-ideal performance.